1. Field of the Invention
The present invention relates to a thin film formation method for forming a non-monocrystal silicon thin film on a substrate in a discharge space.
2. Related Background Art
Capable of implementing a thin film and a large area and composing in a great degree of freedom and controllable for electric and optical characteristics in a wide range, an amorphous semiconductor, e.g. amorphous silicon, attracts an attention recently as materials of various devices, and in particular, amorphous silicon attracts an attention as solar cell materials because of being characterized in that an absorption coefficient near a peak of an energy distribution in solar rays is greater than that of crystalline silicon, its forming temperature is low and a film can be formed directly on a substrate from a source gas by glow discharge.
In solar cells thought highly of as one circle of future new energy measures, the implementation of a lower price and a higher function provides a current significant problem of study and development. Also in the amorphous silicon, those of a considerably high conversion efficiency have become obtainable as solar cell materials, but implementation of a lower price is still insufficient and technical problems such as a larger area or improved film forming rate remain unsolved for mass production.
For the amorphous silicon, generally, generating a plasma by making an RF discharge between a substrate or an electrode supporting the substrate and another electrode and decomposing a source gas mixture of gases containing a silicon compound such as SiH4, hydrogen gas and impurity gases on action of the plasma results in its film formation on the substrate. Several factors can be referred of affecting the film forming rate of amorphous silicon and especially, include a C-S (Cathode-Substrate) distance, a used gas mixture, a pressure and flow rate of the gas introduced into a discharge space, further a substrate temperature and a power and frequency of RF discharge.
As a method for improving the film forming rate of amorphous silicon, contracting the C-S distance is disclosed in Japanese Patent Application Laid-Open No. 5-56850. Besides, Japanese Patent No. 2730693 discloses that contraction of the C-S distance, mixing of He and control of the gas flow rate enables discharge in an atmospheric pressure, thus implementing a high film forming rate. Alternatively, in Japanese Patent Publication No. 7-105354, it is disclosed that the frequency of RF power is changed from 13.56 MHz, frequently used formerly, to a range of 25 to 150 MHz to adopt a VHF discharge and a relation between the C-S distance d and the frequency f is set to 30 MHz/cmxe2x89xa6f/dxe2x89xa610 MHz/cm, thereby enabling a high film forming rate to be obtained.
Only by contracting the C-S distance in forming an amorphous silicon film with the frequency of RF power set to 13.56 MHz, the film forming rate increased, but discharge was instabilized or the irregularity in film forming rate was large though discharge occurs and accordingly a locally thick film was formed. A certain number of such problems remained unsolved to increase the area of an amorphous silicon film. Besides, even if the film forming rate was slightly irregular and a uniform film was formed, a high conversion efficiency is not always obtained surely as solar cell materials.
In consideration of the above circumstances, it is an object of the present invention to provide a method for forming an amorphous silicon thin film easy to making a larger area in which a uniform film forming rate and a high conversion efficiency is obtained.
The present invention made to attain the above object is constructed as follows.
That is, a thin film formation method according to the present invention is characterized by employing a surface of a substrate as one electrode and disposing the surface of the substrate in a distance d (cm) apart from another electrode in a discharge space in which there are positioned at least a pair of electrodes connected to an RF power source, and introducing both a gas containing one or more silicon compounds and hydrogen into the above discharge chamber, setting the product Pd of a film forming pressure P (Pa) and d and hydrogen flow rate M (SLM: flow rate (dm3) per min at the standard state) so as to meet the relation:
80M+200xe2x89xa6Pdxe2x89xa6160M+333
and applying RF power to generate a plasma and to form a non-monocrystal silicon thin film on the substrate in the discharge space.
In such a thin film formation method according to the present invention, the above product Pd and the flow rate L (SLM) of a gas mixture comprising the gas containing one or more silicon compounds and hydrogen are preferably set so as to meet the relation:
67L+200xe2x89xa6Pdxe2x89xa6147L+333.
Furthermore, a thin film formation method according to the present invention is characterized by employing a surface of a substrate as one electrode and disposing the surface of the substrate in a distance d (cm) apart from another electrode in a discharge space in which there are positioned at least a pair of electrodes connected to an RF power source, and introducing both a gas containing one or more silicon compounds and hydrogen into the above discharge chamber, setting the product Pd of a film forming pressure P (Pa) and d and the ratio M/V of hydrogen flow rate M (SLM) to volume V (cm3) of the above discharge space so as to meet the relation:
4xc3x97105dM/V+200xe2x89xa6Pdxe2x89xa68xc3x97105dM/V+333
and applying RF power to generate a plasma and to form a non-monocrystal silicon thin film on the substrate in the discharge space.
In such a thin film formation method according to the present invention, the above product Pd and the ratio L/V of flow rate L (SLM) of a gas mixture comprising a gas containing one or more silicon compounds and hydrogen to volume V (cm3) of the above discharge space are preferably set so as to meet the relation:
3.3xc3x97105dL/V+200xe2x89xa6Pdxe2x89xa67.3xc3x97105dL/V+333.
Besides, in these thin film formation methods according to the present invention, the above distance d lies preferably within a range of 0.5 to 3 cm.